It's best not to start a dog to young but really after the age of 6 months or so they can start to go on the equipment as long as they aren't jumping the full heights. There should be no jumping of full heights until the dog is at least 18 months old 2 prefered.

The place that I volunteer at has puppy classes where the dogs learn foundsation training and some of the basics and some of the equipment as well and then from there they go on to a beginners class where we continue on with what they learned in the puppy class. However we never let any dog jump any more then 4".